In recent years, offshore exploration and production of petroleum products has been conducted in arctic regions such as northern Canada and Greenland. Offshore exploration and production in these areas have presented particular problems not found in more temperate climates. One of the major problems encountered in arctic offshore waters is the presence during much of the year of large moving ice masses such as ice floes or icebergs. Propelled by wind and sea currents, these large ice masses--ice-bergs having a depth of several hundred feet and a weight of several million tons are not uncommon in certain arctic regions--may travel considerable distances per day at relatively high velocities. Such large moving ice masses possess large amounts of kinetic energy and can impose very great forces on any offshore structure in their path, possibly causing extensive damage to the structure or even catastrophic failure of the structure. Therefore, offshore structures located in this type of environment must either be built strong enough to withstand the forces associated with a large impinging ice mass or must be able in some manner to reduce the forces imposed on the structure by the impinging ice mass.
The cost of building a structure strong enough to withstand such forces is generally considered to be prohibitively expensive; therefore, various concepts have been proposed for reducing the forces imposed on the structure by the impinging ice mass. One of these designs is described in U.S. Pat. No. 3,283,515, which discloses the use of a ring of outer support members encircling the inner support members of the offshore structure. The inner support members provide the main vertical support for the structure's platform while the outer support members prevent large ice masses from colliding with the inner support members. Means are also provided for damping the vibrations resulting from the impact of the ice mass with the outer protective support members.
Another approach for minimizing the impact of moving ice masses with an offshore platform is disclosed in U.S. Pat. No. 3,348,382 wherein the platform's vertical support columns are only cross-braced in the region from the water bottom to about 10 to 20 feet below the waterline and from about 10 feet above the water line to the bottom of the platform deck. Since all cross-bracing is eliminated throughout the vertical height of a so-called typically sized ice floe, the impinging ice floe will only come into contact with the vertical support columns of the platform. Therefore, the only force imposed on the structure by the ice floe is that associated with the breaking and shearing of the ice floe as it moves into contact with the vertical columns.
Still another approach to this problem is illustrated in U.S. Pat. No. 3,436,920, which discloses an iceberg protective system in which buoy-supported cables are arranged around an offshore platform at a considerable distance from the platform. This fending system, which may be augmented by drag anchors and crash tubes, is designed to effectively stop the forward motion of the iceberg thereby preventing the iceberg from colliding with the platform.
The systems proposed heretofore either involve the use of specially designed platforms, which may entail increased cost and time of construction, or external protective systems which are designed to prevent the major part of an impinging ice mass from colliding with an offshore structure. Although these systems offer some protection against impinging ice masses, they are not particularly suited for use in deeper waters which may contain relatively larger ice masses in that these systems become extremely complicated and costly to construct and repair. There is then a need for a system for reducing the forces imposed on an offshore structure by relatively large impinging ice masses, such a system being particularly adopted for use in deeper waters.